1. Field of the Invention
The present invention relates to a plasma-assisted sputter deposition system, and more particularly, to a plasma-assisted sputter deposition system for depositing thin films on a wafer, in which a wafer holder is at rest during the film deposition, with a great uniformity and a great accuracy of film thickness controllability.
2. Description of the Related Art
Magnetron DC or RF sputtering systems are in wide application to deposit thin films on substrates used in semiconductor industry. One of the major requirements in depositing films on substrates, for examples, on Si wafers, is the film uniformity. In order to deposit films with a greater uniformity, a DC sputtering system with an angled electrode with respect to wafer surface has been invented and in use (JP-A-2002-167661 and JP-A-2002-296413). However, this system has some problems in controlling the substrate temperature, re-sputtering of deposited film and particle reduction. These problems are explained in detail with reference to FIG. 5.
FIG. 5 shows a cross sectional view of a DC sputtering system with an angled electrode. The reactor 100 is comprised of an electrode 101, a wafer holder 102, gas inlets 103 and a gas outlet 104. The electrode 101 is made of a metal, for example, Al, Ti, Ta etc. that needs to be sputtered and deposited on a wafer 112. The electrode 101 is electrically isolated from the reactor 100 using a dielectric material 105. Generally, on the upper surface of electrode 101 a plurality of magnets 106 with some specific arrangement has been placed. Further, magnets 106 arranged to the electrode 101 are rotated around an off-axis or the central axis of the electrode 101. A round arrow 121 shows the state of rotation movement around the central axis. The electrode 101 is connected to a DC power source 107.
Usually, the wafer holder 102 is comprised of a metal electrode 108, dielectric material 109, side-walls 110 and a shaft 111. The shaft 111 is connected to an electrical motor in order to rotate the wafer holder 102 on its central axis as shown by a round arrow 122. The electric motor is not shown in the figure.
DC plasma is generated within the reactor 100 by applying a DC current to the electrode 101 while maintaining a low pressure inside the reactor 100. Owing to the higher negative voltage of the electrode 101, ions in the plasma are accelerated to the electrode 101 and sputtered. These sputtered atoms then travel through the plasma and deposit on the wafer 112 and other surfaces, which are exposed to plasma.
The sputtered-atom flux coming from the angled electrode 101 is not uniform on the wafer surface. In order to get a uniform film on the wafer surface, the wafer holder 102 is rotated around its central axis. This results in a uniform film. All the problems in the above-explained PVD system come due to the wafer holder 102 being rotated.
The first problem is that wafer temperature during the film deposition cannot be controlled. This is because incorporation of a liquid-base cooling mechanism for the metal electrode 108 is difficult. Incorporation of a heating mechanism for the metal electrode 108 is also difficult. These difficulties are mechanical difficulties and arise due to the rotation of wafer holder 102. Deposition of films at a controlled temperature is of importance in obtaining films with desired physical and electrical properties. For example, deposition of Cu films as a seed layer for electroplating process has to be carried out at a temperature below zero for better surface smoothness.
The second problem is that fabrication of electrostatic chuck (ESC) on the metal electrode 108 to clamp the wafer by using electrostatic force is difficult due to the complexity in designing electrical connections. Wafer clamping by using the ESC is important when control on wafer temperature is needed during the film deposition process.
The third problem is that the supply of a rf current to the metal electrode 108 is also not easy due to the difficulty in making electrical connection to the rotating wafer holder 102. Application of the rf power to the metal electrode 108 is needed if the deposition has to be carried out with the bombardment of soft-ions or if the deposited film is needed to be re-sputtered.
The forth problem is the generation of particles within the reactor due to continuity of rotation of the wafer holder 102. The rotation of the wafer holder 102 causes vibration of its outer parts and peripheral parts. This eases delaminating of deposited film from the vibrating surfaces, which finally causes particle contamination on the wafer surface.
In addition, as the related arts concerning cathode or target of doughnut-shaped, documents such as JP-A-7-126847 and JP-A-5-189762 may be cited.